1. Field of the Invention
The present invention relates to an image forming apparatus including at least one image carrier, and an endless transfer belt for carrying an image or conveying a transfer material.
2. Description of the Related Art
Hitherto, a direct transfer method and an intermediate transfer method have been employed as a method for transferring toner images from a photosensitive member to a transfer material in an image forming apparatus that scans an electrophotographic photosensitive member with a laser beam that is modulated based on image information, for example.
According to the direct transfer method, a belt-like unit for conveying a transfer member conveys a transfer material through a transfer position to directly transfer toner images from a photosensitive member to the transfer material. The intermediate transfer method primarily transfers toner images on a photosensitive member onto a belt-like intermediate transfer member at a primary transfer position, and transfers the primary transfer images onto a transfer material at a secondary transfer position.
The endless transfer belt used as a transfer material conveying member or an intermediate transfer member in the image forming apparatus is stretched over a plurality of rollers and rotated in a predetermined direction (sub-scanning direction). However, a so-called one-sided travel sometimes occurs due to non-parallel movements of the plurality of rollers or variations in outer diameter among the rollers. In that case, the endless transfer belt moves while deviating from its normal direction in an axial direction of the roller (main-scanning direction, i.e., exposure-scanning direction), which is orthogonal to the moving direction.
The one-sided travel causes a phenomenon that the belt moves in the main scanning direction and toner images are not transferred onto a proper position on the transfer material. The phenomenon is called margin deviation. Further, in an inline-type color image forming apparatus in which an endless transfer belt circulates through a plurality of transfer positions to superimpose toner images on one another, a color drift may occur due to the one-sided travel. The color drift is a phenomenon that transferred toner images are misaligned in a main scanning direction.
To address the above problem, Japanese Patent Application Laid-Open No. 2005-338111 discusses a method for changing a timing at which an optical scanning unit starts scanning a photosensitive member in a main scanning direction based on positional information of an endless belt in the main scanning direction. Thus, positions of toner images on the photosensitive member can be adjusted in the main scanning direction according to a position of the belt in the main scanning direction.
In general, the endless transfer belt does not deviate from its original course to a main scanning direction while moving in parallel to the sub-scanning direction, but the transfer belt is slightly skewed in a sub-scanning direction when the deviation occurs. Moreover, the belt is mechanically and electrostatically sandwiched between a photosensitive member and a transfer unit at a transfer position, so that an inclination angle of the belt to the sub-scanning direction might vary between an upstream side and a downstream side from the transfer position.
Thus, in such an image forming system which adjusts a timing to start scanning in a main scanning direction based on positional information of an endless transfer belt in a main scanning direction, if the scanning start timing is adjusted only based on positional information of the belt on an upstream side from a transfer position, it is difficult to adjust positional deviation of the belt in the main scanning direction. For example, a difference between a main scanning direction of the belt at a sensor detection position that is apart from a transfer position and a main scanning direction of the belt at the transfer position directly leads to a color drift.